dav1d: updated to 0.2.1 / added meson opts to disable tools & tests

[5schatten]

Tested with:

• https://www.elecard.com/videos
• http://download.opencontent.netflix.com/?prefix=AV1/Chimera/

My J3455-ITX was able to handle this 1080p / 8bit video file at 24Hz which equals roughly performance wise an Intel® Core™2 Quad Processor Q6600 so I guess older cpus with SSSE3 support should be able to handle this video specs as well.

Additional updates:

• ffmpeg: updated AV1 support patch for dav1d 0.2.1 -> backported upstream libdav1d.c
• ffmpeg: fixed multithreaded sw decoding for AV1 thx to @lrusak

Release notes:

• https://code.videolan.org/videolan/dav1d/tags/0.2.1

dav1d 0.2.1 'Antelope', the fast and small AV1 decoder

This is the second release of dav1d, the fast and small AV1 decoder,
codename 'Antelope'.

It still supports all AV1 features and all bitdepths, from 8 to 12bits,
and decodes more samples than before.

While still being the fastest decoder on AVX-2 CPUs, dav1d is now also
faster on ARM, ARM64 and SSSE3 CPUs in normal loads.
0.2.x is around 2x/2.5x faster on SSSE3 and achieves 1080p at 80fps
on an iPhone XS

0.2.1 is a small bugfix release above 0.2.0 to fix build issues, and
increase a bit the speed of SSSE3